Machine Vision ENT 273 Regions and Segmentation in Images Hema C.R. Lecture 4

Hema ENT 273 Lecture 4 2 Road Map Regions Region Segmentation Thresholding Methods Region Representation Region Merging Region Splitting Region Growing

Hema ENT 273 Lecture 4 3 Regions A region is a group of connected pixels with similar properties Regions may correspond to objects in a scene and hence are important in image interpretation

Hema ENT 273 Lecture 4 4 Segmentation –A method to partition an image into sub images is called regions –Each region is an object candidate –Segmentation is a very important step in understanding images

Hema ENT 273 Lecture 4 5 Region Segmentation Thresholding is the simplest region segmentation technique Simple segmentation is conversion of a grayscale image to a binary image where image is partitioned into two sets. Automatic Thresholding

Hema ENT 273 Lecture 4 6 Automatic Thresholding A thresholding scheme that uses knowledge of objects such as Intensity levels of objects Size of objects Fraction of image occupied by objects Types of different objects in image to select a proper threshold value for each image without human intervention is called an automatic thresholding scheme

Hema ENT 273 Lecture 4 7 Common Approaches to Automatic Thresholding P-tile method Mode method Iterative Method Adaptive Method Variable Method Double Thresholding Method

Hema ENT 273 Lecture 4 8 Common Approaches to Automatic Thresholding P- Tile Method –Use size or area of desired object to threshold an image, threshold chosen to assign p percent of pixels to the object. Mode Method –When objects and background have two different gray levels, then threshold can be determined from the valley points of the histogram Iterative Threshold Selection –Starts with approximate threshold and successively refined based on results

Hema ENT 273 Lecture 4 9 Common Approaches to Automatic Thresholding Adaptive Thresholding –Used in cases of uneven illumination, analyses sub images to obtain threshold for sub image Variable Thresholding –Used in uneven illumination, approximates intensity values of image by a simple planar function to find threshold of the image. Double Thresholding –Uses two thresholds to segment an image, first T to segment core image and second T to select pixels connected to core.

Hema ENT 273 Lecture 4 10 Region Representation Three Types –Array Representation –Hierarchical Representation –Symbolic Representation

Hema ENT 273 Lecture 4 11 Array Representation An array of the same size of the original image is used to indicate the region to which a pixel belongs Membership arrays called masks are used to indicate which pixels belong to that region E.g.. Background pixels in a binary image

Hema ENT 273 Lecture 4 12 Hierarchical Representation Hierarchical representation make it possible to use algorithms which decide a strategy for processing on the basis of relatively small quantities of data. Allows representation of images in multiple resolutions –Pyramid An image having one degree smaller resolution in a pyramid contains four times less data, so that it is processed approximately four times as quickly Pyramid

Hema ENT 273 Lecture 4 13 Hierarchical Representation –Quad Tree Quad tree is an extension of pyramids for binary images Contains three nodes –White –Black –Gray Obtained by recursive splitting of an image When all points in sub region are either black or white then region is no longer considered for splitting Quad Tree

Hema ENT 273 Lecture 4 14 Split and Merge Intensity based segmentation results in too many regions Images are to be refined or reformed e.g. –Merge adjacent regions with similar characteristic –Remove questionable edges –Use topological properties of the regions –Use shape information of objects –Use semantic information about the scene

Hema ENT 273 Lecture 4 15 Combines regions considered to be similar Important - to determine similarity between regions Region based merging –Two approaches Compare mean intensities with some threshold Probability distribution of intensities Removing weak edges –Common boundary between two regions is dissolved if boundary is weak and gray value of resulting boundary is not much effected. –A weak boundary is one for which the intensities on either side differ by less than a threshold T Region Merging

Hema ENT 273 Lecture 4 16 Region Splitting If some property of a region is not constant then the region should be split Important : where to split? Segmentation can be refined by combining split and merge operations

Hema ENT 273 Lecture 4 17 Region Growing Selects a seed region in an image Merges adjacent similar regions to the seed Repeats till all regions except regions smaller than a given criteria

Machine Systems ENT 273 End of Lecture 4